Rectangular waveguide microwave amplitude modulator with a planar resistive attenuator extending along ferromagnetic rod



June 8, 1965 c. BOWNESS 3,188,582

RECTANGULAR WAVEGUIDE MICROWAVE AMPLITUDE MODULATOR WITH A PLANARRESISTIVE ATTENUATOR EXTENDING ALONG Original Filed Sept. 28, 1959FERROMAGNETIC ROD 4 Sheets-Sheet l INVE/VTUR COL IN BOW/V553 A TTOP/VE)"June 8, 1965 c. BOWNESS 3,188,532

RECTANGULAR WAVEGUIDE MICROWAVE AMPLITUDE MODULATOR WITH A PLANARRESISTIVE ATTENUATOR EXTENDING ALONG FERROMAGNETIC ROD Original FiledSept. 28. 1959 4 Sheets-Sheet 2 FERRITE D/AMETER 0.200 m VANE RESIST/WT)20o FREQUENCY Q KMC/S 20 12.4 S F/ G. 3 I 0 B E t /0 //.4 Y

l I 25 50 75 APPL/[D F 1610- ORSTEDS FERRITE DIAMETER 0.250 VAA/ERL'S/ST/V/ 7) 200% FREQUENCY g KMC/S 20 4 g m F G 4 R 3 E E /0 11.4 k K25 5O 75 APPLIED F/ELD ORSTE'DS FREQUENC) 9.4 KMC/S PEER/TE .D/AMETER gVANE RESIST/V/TY 200% INCHES 20 g 0.250 F 5' R 3 E k /0 i; 0.225 0,200f0: l

A TTORNE) 25 5'0 APPL/ED F/ELD ORSTEDS June 8, 1965 Original c BOWNESS3,188,582

RECTANGULAR WAVEGUIDE MICROWAVE AMPLITUDE MODULATOR WITH A PLANARRESISTIVE ATTENUA'I'OR EXTENDING ALONG FERROMAGNETIC ROD Filed Sept. 28,1959 4 Sheets-Sheet 5 FREQUENCY 10.4 hMC/S VA/VE REs/sT/V/Ty 200%FERR/TE DIAMETER INCHES 50 75 APPLIED FIELD ORSTEDS FREQUENCY 9.4 KMC/SFERR/TE DIAMETER 0.250 APPL/[D FIELD 70 OPSTEDS mo 200' VANE RES/5T/V/TY- OHM S/SQU ./2.4 KMC/S i I I I L I l l l //V VE/V TOR /a 20 I 5aAPPLIED F/L.D-,;-0PSTED5 COL/N BOW/V535 ATTORNEY 3,188,582 R WITH C.BOWNESS June 8, 1965 RECTANGULAR WAVEGUIDE MICROWAVE AMPLITUDE MODULATOA PLANAR RESISTIVE ATTENUATOR EXTENDING ALONG FERROMAGNETIC ROD 4Sheets-Sheet 4 Original Filed Sept. 28 1959 l/VVE'NTOP COL/N BOWNESSw/MM United States Patent 3,188,582 REQTANGULAR WAVEGUWE WQRBWAVE, AM-PLHTUDE MtlDULATOR lVlTH A PLANAR RE- Si'EsTiVE ATTENUATGR EXTENDINGALGNG FERROlv EAGNETIC RG11) Colin Eowness, Weston, Mass, assignor toRaytheon Company, Waltham, Mass, a corporation of Delaware Continuationof application Ser. No. $42,69tl, Sept. 28, 1959. This application last.19, 1964, er. N0. 339,ll53 6 Claims. (Cl. 333-24.2)

This is a continuation of my copending application, Serial No. 842,690,filed September 28, 1959, now abancloned.

The present invention relates to the art of modulating high frequencyelectromagnetic energy. More particularly, this invention relates toattenuation and modulation of high frequency electromagnetic energy aspropagated in waveguides and other transmission lines.

It is often required to alter the amplitude of a microwave signal in awaveguide by means of some external control. Amplitude modulation ofmicrowave energy in a transmission line may be obtained, for example, byalternately introducing and removing attenuation. This may beaccomplished mechanically by the insertion and removal of a resistivevane in the plane of electric polarization at the desired rate. As themodulation frequency increases mechanical problems multiply and a moreelegant prior art solution contemplates the use of ferrite in a variablemagnetic field. Two commonly used methods, for example, are the use offerromagnetic resonance phenomenon and the use of Faraday rotation in acircular waveguide. Use of these techniques requires means for matchinginput and output to standard rectangular waveguide and acceptableresults may be obtained only when this problem is solved. The use offerrite in a variable magnetic field makes it possible to rotate theplane of electric polarization in the transmission line into a fixedresistive element or to distort the electric field in such a way that acomponent is induced in the plane of the resistive element. Mostlow-power ferrite amplitude modulators in the prior art rely for theiroperation on rotation of the plane of polarization. A typical reciprocalamplitude modulator of this type is shown in FIG. 1 for purposes ofcomparison with the present invention. The rotation of polarization 0 isproportional to the magnetic field and the transmitted field isproportional to cos 0.

Such Faraday rotation may also be considered as being due to a uniformdistributed coupling between the vertically and horizontally polarizedwaves in the circular section. This distributed coupling is betweentransverse components of the RF magnetic field Within the ferrite and isdue to the electron spins processing about the direction of DC.magnetization. When thetwo modes of propagation have the same phasevelocities, it is possible to transfer all the energy from one mode tothe other. The process is continuous and in a long line the energy willcycle from one mode to the other. The period depends upon the couplingcoetficient, which is a function of the applied magnetic field.

This approach is convenient when considering a square or circularwaveguide in which an axial ferrite rod is used as the couplingmechanism between the dominant TE modes. In square or circular waveguidethe phase velocities are equal, and in a given length of line theproportion of energy coupled from one mode to the other will ice dependupon the applied field. In standard rectangular waveguide having unequalcross-sectional dimensions, within the recommended frequency range, thehorizontally polarized modes are cut off. The presence of a ferrite rodwill alter the transmission constants of the waveguide, but if thediameter of the rod is sufficiently small to preclude propagation in thehorizontal mode at the frequencies of interest, any energy radiated intothis mode must be re-radiated in the vertically polarized mode. theeffect of such re-radiated energy is a change in the phase constant ofthe transmission line as a whole, hence phase shift may be controlled inthis manner by the applied magnetic field. The operation of reciprocalphase shifts is Well known in the art and is discussed in A NewTechnique in FerritePhase-Shifting for Beam Scanning of MicrowaveAntennas by F. Reggia and E. G. Spencer, Proceedings of the IRE, volume45, pp. 15104517, November i957.

Proceeding from the hypothesis given immediately hereinabove, inaccordance with the present invention the difficulties and disadvantagesnoted here'inbefore and associated with prior art amplitude modulatorsare overcome by placing parallel to the broad or wide walls of astandard rectangular waeguide a resistive vane or vanes located alongthe length of and preferably in contact with a ferrite rod which in turnis located on the axis of the rectangular waveguide. By providingresistive rectangular vanes in accordance with the invention an appliedmicro- ,wave signal may be attenuated by application of a longitudinalmagnetic field, thereby resulting in an amplitude modulator havingsubstantial advantages over known prior art devices. Among the manyadvantages of the present achieve attenuations of 30 decibels or moreover narrow frequency bands by use of the technique disclosed herein andif a ridged waveguide technique is used, for example, to broadband thedevice an amplitude modulator may be made capable of about fifty percentmodulation over the entire recommended frequency range of X-bandwaveguide (8.2 to 12.4 limo/S.) having an attenuation for a givenapplied magnetic field that is substantially constant across thisfrequency band.

Theseand other objects and features of the invention, together withtheir incident advantages, will be more readily understood andappreciated from the following detailed'description of the preferredembodiments thereof selected for purposes of illustration and shown inthe accompanying drawings in which:

FIG. 1 is a perspective view of a prior art amplitude modulator;

FIG. 2 is a perspective view, with parts broken away for clarity, of oneembodiment of the invention;

FIGS. '3-7 are graphic representations of operating characteristics ofthe invention;

of the invention;

a 0.200 inch diameter ferrite rod in rectangular waveguide.

With reference now to FIG. 1 which is exemplary of conventional priorart devices, a rectangular input waveguide 11 constructed, for example,of copper has a transverse dimension 12 less than a half-wave long atthe operating frequency and the other transverse dimension 13 is greaterthan a half-wave long at the operating frequency. A cylindricalwaveguide 14 is electrically and mechanically coupled to the input guide11 by a conventional transition section 15 as shown. A ferrite rod 16 islocated on the axis of the cylindrical waveguide 14. A coil 17 surroundsthe cylindrical waveguide 14 and ferrite rod 16 as shown. A rectangularoutput waveguide 18 identical in configuration and orientation to theinput waveguide 11 is electrically and mechanically coupled to theoutput end of the cylindrical waveguide 14 'by a transition section 19identical to transition section 15.

A bias magnetic field may he provided by a suitable bar magnet (notshown) and microwave electromagnetic energy characterized by an electricvector having a polarity in the direction as shown at 21 is coupled tothe guide 11 in well-known manner. A suitable modulating voltage isapplied to the coil 17 to cause an alternating magnetic current to flowlongitudinally through the ferrite rod 16. In accordance with theso-called Faraday effect as applied to microwave energy, the magneticcurrent parallel to the rod 16 causes the electric vector of the energyto rotate in accordance with the variations of the current in the coil17.

The amount of rotation that takes place is also a function of the lengthand diameter of the ferrite rod. In most prior art embodiments, theelectric vector is rotated :90 degrees about a mean position determinedby the steady state transverse magnetic field. That is to say, it

rotates 90 degrees clockwise and 90 degrees counterclockwise inaccordance with the maximum amplitude in opposite polarities of themagnetic current.

As is well known in the art, the guide 18 propagates that component ofelectromagnetic energy having an electric vector polarized in thedirection as shown at 22. As is Well known the rotation of polarization6 is proportional to the magnetic field produced by the modulatingvoltage and the transmitted power accepted by guide 18 is proportionalto cos since the orientation of guides 11 and 18 about the longitudinalaxis of the device is identical.

Coplanar vanes 2324 are provided as shown in FIG. 1. The vanes 2324 arerelatively thin rectangular sheets of resistive material attached alonga diameter of the guide 14 adjacent'the ends of therod 16 parallel tothe direction of the propagation of the energy therein and perpendicularto its electricvector. Upon application of a modulating voltage to thecoil 17, an alternating magnetic current passes longitudinally throughrod 16. Energy passing rod 16 is characterized by a rotating vector 25having instantaneous positions at times. perpendicular and at timesparallel to vane 24 (and vane 23). Since the vane 24 (or vane 23)permits only that component of the energy having an electric vectorperpendicular thereto to, pass (as shown at 22), modulation is obtained.

With reference now to FIG. 2 an embodiment of an X- band reciprocalmodulator constructed in accordance with the present invention comprisesa ferrite rod 31 tapered at both ends 32-33 to give a good impedancematch over a wide range of frequencies and magnetizing fields issupported by a suitable dielectric material 34, such as Styrofoam, onthe longitudinal axis of a standard copper rectangular waveguide 35having a and b dimensions suitable for X-hand transmission such as, forexample, respectively one inch and one-half inch. Resistive vanes 36-37increases.

parallel to the wide sides 38-39 of the waveguide 35 and extendingoutwardly from the ferrite rod are each carried in a long slot 41. Othersuitable means such as disks, metal pins and the like may be used tosupport the rod 31. An axial magnetic field is provided by solenoid 42to which is coupled the modulating signal. In an embodiment thatoperated satisfactorily at X-band frequencies (8.2 to 12.4 krnc./s.) theferrite rod 31 was composed of R-151, a commercially availablemagnesium-manganese ferrite, whose dielectric and low-field losses aresmall at X-band frequencies. The ferrite rod 31 was three inches longand had a diameter in the range of .150 to .250 inch and was providedwith a one inch taper 32-33 at its ends. The solenoid 42 wasapproximately two inches long and the resistive vanes 36-37 were abouttwo inches long, .200 inch wide and were located in slots 41 that wereabout 0.3 inch deep. With the one inch long tapers the VSWR rarelyexceeded 1.20. The electrical characteristics as functions of frequency,rod diameter, and vane resistivity are shown in FIGS. 3, 4, 5, 6 and 7.

FIG. 3 shows typical curves of attenuation versus applied axial fieldfor a 0.200 inch diameter rod and various frequencies. The surfaceresistivity of the vanes was 200 ohms per square where ohms per squareis determined in conventional manner for measuring resistivity. At lowlevels of internal field, coupling into the resistive vane isapproximately proportional to the magnetization of the ferrite.Therefore, the attenuation curves as shown in FIG. 3 take on the generalform of a magnetization curve and since the longitudinal demagnetizingfactor of the rod is small, the ferrite saturates at a low applied fieldand the attenuation tends towards a limit as shown.

As may be seen by reference to FIG. 3, the attenuation increases rapidlywith frequency, and this may be explained in the following manner. TheWaveguide is partially loaded with ferrite having a high dielectricconstant and, as the frequency increases, the electromagnetic energyconcentrates more and more in the rod, which acts as a dielectricwaveguide. Moreover, the impedance of a TB mode in a waveguide decreaseswith increasing frequency, and consequently the transverse RF magneticfield Both effects result in an increase in the transverse RF magneticfield within the rod, and therefore the ferrite couples a greaterintensity of RF electric field into the resistive vanes 36 and 37.

In order to clearly show the effects and the limits of variations of thediameter of the ferrite rod the characteristics of a 0.250 inch diameterrod are shown in FIG. 4. With the increase in rod diameter more energytravels within the rod at a given frequency, and it is suggested thatthe curve for 9.4 kmc./s. in FIG. 4 marks the beginning of a transitionfrom nonpropagation to a propagation of a wave polarized parallel to thegreatest crosswise dimension of the waveguide. The existence of such amode suggests one explanation of the peaks in attenuation on the 10.4kmc./s. and 11.4 kmc./s. curves. The transfer of energy from one mode tothe other is a function of both the length of the rod and themagnetization. When the combination of these factors is such that mostof the remaining energy arrives at the end of the ferrite sectionpolarized parallel to the widest crosswise dimension of the waveguide,it must be reflected, as it cannot propagate in the unloaded waveguide.The result would be a peak of attenuation at this magnetization.

In dielectric waveguides the ratio of energy in the dielectric to totalenergy in the waveguide is determined by the ratio of rod diameter towavelength. Therefore, an increase in rod diameter has approximately thesame effect on energy concentration as an increase in frequency. This isshown in FIGS. 5 and 6 in which the curves are similar in form to thosein FIGS. 3 and 4. However, the variable in each set is now diameterinstead of frequency. The results indicate that a given characteristicmay be obtained in a range of frequencies by a suitable choice of roddiameter.

In FIG. 7 a curve is given of attenuation as a function of vaneresistivity for a 0.250 inch diameter rod operating at a field of 70oersteds at a frequency of 9.4 kmc./s. Inspection of FIG. 7 will showthat maximum attenuation occurs at a surface resistivity of 160 ohms persquare. It has been found that a reduction in the width of the vaneswill reduce the surface resistivity at which maximum attenuation occurs.The shape of the curve will not change substantially with frequency.

An increase in length of the vanes and of the cylindrical center sectionof ferrite, from, for example, one to two inches, will giveapproximately twice the previous value of attenuation, as is to beexpected.

The depth of the slot in the ferrite for receiving the resistive vanesmay be increased, for example, from 0.030 to 0.060 inch withoutsignificant change and no significant change is obtained for vaneshaving a width in excess of one-fourth the a or largest dimension of thewaveguide.

To consider possible applications of the present invention, reference isnow made to FIG. 5. The curves shown in FIG. 5 indicate that a 0.250inch diameter rod is satisfactory for use in an amplitude modulator at9.5 kmc./s. A DC. biasing field would be required to give operation on arelatively linear portion of the characteristics. As the field levelsrequired are low, an A.C. modulation power of a fraction of a watt wouldbe sufficient to give 30 percent modulation with low harmonicdistortion. It is also possible to operate the device with no D.C.biasing field, in which case the modulationenvelope varies at twice themodulation frequency, but this will introduce distortion.

If the slope of the attenuation characteristic and maximum attenuationreached are increased over the values given hereinabove by lengtheningthe center section, the device may be suitably used for pulsedmodulation or onoif switching. With a one inch increase in length, apulse field of 50 oersteds will give more than about 35 db ofattenuation. The current regulation during the pulse need not becritical as the slope of the attenuation characteristic is small beyondthe saturation point of the ferrite.

For those skilled in the art who will utilize the invention in one ormore or" the many applications for which it is suitable, attention iscalled to the fact that a limitation of the device is the sensitivity ofthe characteristics to a change in frequency.

A modification of the invention is shown in FIGS. 8 and 9 whichcomprises a broadband modulator. As shown in FIGS. 8 and 9 a ferrite rod51 and associated resistive vanes 52-53 substantially identical incomposition and configuration as those described hereinabove, issupported on the longitudinal axis or" a rectangular waveguide 54 havingtwo coplanar metal ridges 55-56 integral with the broad walls of thewaveguide 54. The ferrite rod 51 is bonded as by epoxy resin to each ofthe ridges 555 which provide excellent mechanical support for theferrite rod 51.The ridges 55-56 are tapered to give the requiredimpedance match over the desired'bandwidth. Solenoid 57 provides thenecessary axial magnetic field and a Mu- Metal shield 58 surrounds thedevice to screen it from stray external fields. It has been previouslythought that the frequency sensitivity of devices of the type hereconcerned was due to two things; the change in TE wave irnpedances withfrequency, and the change with frequency in the ratio of energytraveling within and without the ferrite rod. Both of these elfects areconsiderably reduced by the introduction of broadbanding means such asridges in the waveguide.

With modulation frequencies up to, for example, kc./s., a thick-walledmetal waveguide is impractical because eddy currents induced in theshort-circuited turn formed by the waveguide produce a bucking magneticfield which results in a severely attenuated modulated field infrequency in excess of a few kilocycles per second.

The attenuation characteristics of the modulator in g ridged waveguideare given in FIG. 10 together with two curves for a 0.200 inch diameterrod in rectangular waveguide for the purposes of comparison. Using abiasing field of about 10 oersteds, and modulating with a peak-to peakfield of 4 oersteds, a peak to trough modulation of approximately 0.8d.c. may be obtained, with an average insertion loss of about 1.5 dbmaximum at 12.4 kmc./s. When RF losses in the plastic waveguide areincluded, the average insertion loss will not exceed about 1.9 db.

It may now be obvious that there has been described a simple, effectivelow-power amplitude modulator that does not requi e bandwidth limitingtransitions, that is inexpensive to manufacture, and that can be used,for example, as a conventional amplitude modulator or as an on-ofi?switch. Since the modulator utilizes a rectangular waveguide, impedancematching over a wide band is easily obtained by tapering the ferrite roddisposed therein and variation of one parameter, the rod diameter, thenpermits a wide range of attenuation characteristics to be selected toprovide a very efiicient broadband and high frequency amplitudemodulator.

In all cases, it is to be understood that the above-describedarrangements are simply illustrative of a small number of the manypossible specific embodiments which can represent applications of theprinciples of the invention. .Numerous and variedother arrangements canreadily be devised by those skilled in the art without departing fromthe spirit and scope of the invention.

What is claimed is:

1. A microwave modulator device comprising the combination of arectangular waveguide having a longitudinal axis, means for applying alongitudinal magnetic field within the waveguide, a rodlikeferromagnetic element supported within the waveguide and having itslongitudinal axis extending coaxially with the axis of said wave guide,and planar resistive means extending along the length of and supportedby said rod and located wholly in a plane which is disposed parallel tothe longer side walls of the waveguide, which plane extends through theferromagnetic element, said planar resistive means terminating in endwall portions spaced from the narrower sidewalls of the Waveguide.

2. device as set forth in claim 1 wherein the ferromagnetic element issupported within the waveguide upon a body of dielectric material.

3. A device as set forth in claim 1 wherein the ferromagnetic elementhas tapered end portions to provide impedance matching over a wide rangeof frequencies and magnetic fields, and the planar resistive meansextends between'said tapered end portions.

4. A' device as set forth in claim 1 wherein the means for applying alongitudinal magnetic field embodies a solenoid coil which encircles thewaveguide.

5. A device as set forth in claim 1 wherein the planar resistive meanscomprises at, least one planar element hav ing at least a portionthereof insertedwithin the body of the ferromagnetic element.

6. In combination, a waveguide having a first electrically effectivecross-sectional dimension longer than a halfwavelength at the desiredoperating frequency and a second electrically effective cross-sectionaldimension.

shorter than a half-wavelength at the desired operating frequency, firstand second inwardly-extending conductive ridges carried respectively byoppositely disposed inner surfaces of said waveguide, an elongateferrite rod disposed intermediate said ridges and along the longitudinalaxis of said waveguide, said rod having two oppositelydisposed groovessubstantially parallel to said first crosssectional dimension, first andsecond vanes of resistive material, each fixedly disposed in one of saidgrooves and extending outwardly from said rod, substantially parallel tosaid first cross-sectional dimension, each said vane'being coextensivewith at least the center portion of said rod and defining an end wallspaced from said second crosssectional dimension, means bonding said rodto at least one of said ridges and holding said rod and said vane inspaced relationship to said waveguide, and means for applying a magneticfield to said ferrite rod substantially parallel to the axis of saidwaveguide.

References Cited by the Examiner UNITED STATES PATENTS 3,018,454 1/62Sferrazza 333 s1 8 FOREIGN PATENTS 123,581 11/59 USSR.

OTHER REFERENCES Bowness et aL: Low Power Modulator, IRE CanadianConvention Record, Oct. 10, 1958, pages 475-479.

HERMAN KARL SAALBACH, Primary Examiner.

1. A MICROWAVE MODULATOR DEVICE COMPRISING THE COMBINATION OF A RECTANGULAR WAVEGUIDE HAVING A LONGITUDINAL AXIS, MEANS FOR APPLYING A LONGITUDINAL MAGNETIC FIELD WITHIN THE WAVEGUIDE, A RODLIKE FERROMAGNETIC ELEMENT SUPPORTED WITHIN THE WAVEGUIDE AND HAVING ITS LONGITUDINAL AXIS EXTENDING COAXIALLY WITH THE AXIS OF SAID WAVEGUIDE, AND PLANAR RESISTIVE MEANS EXTENDING ALONG THE LENGTH OF AND SUPPORTED BY SAID ROD AND LOCATED WHOLLY IN A PLANE WHICH IS DISPOSED PARALLEL TO THE LONGER SIDE WALLS OF THE WAVEGUIDE, WHICH PLANE EXTENDS THROUGH THE FERROMAGNETIC ELEMENT, SAID PLANAR RESISTIVE MEANS TER- 